1. Field of the Invention
The present invention relates to a method of imaging and erasing an erasable printing form.
2. Description of the Prior Art
From the textbook, xe2x80x9cTechnologie des Offset-Druckxe2x80x9d {Technology of Offset Printing] by R. Riedl, D. Neumann, J. Teubner, (Leipzig), 1989 (1st Edition), it is already known to charge an aluminum printing plate, which bears a photosemi-conductive layer on its surface, electrically as a whole and then expose it in accordance with a picture which is to be printed. At the exposed places of the printing form, the charges flow off while they remain on the non-exposed places. Charged dry or liquid toner particles of opposite electrical charge are then applied by a roller. The toner particles are applied only to the non-exposed places of the printing form. The toner particles which have been applied are then fixed by heat.
European reference EP 0 099 264 A2 discloses a method for illustrating a printing form with dry toner particles. Here, the surface of a substrate is covered in its entirety with electrostatically charged dry toner particles whereupon these particles are melted by laser light in the picture regions so that they adhere firmly to the substrate.
EP 0 580 394 A2 discloses a method for the imaging of a lithographic plate by applation, in which portions of a plastic layer corresponding to a picture to be printed are removed by laser radiation.
The object of the present invention is to provide a method for imaging and erasing an erasable printing form.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in a method of imaging and erasing an erasable printing form, which includes the steps of electrically charging the printing form on its entire surface so that liquid toner particles, which have either individual charges opposite the charges of the printing form or dipole or multi-dipole moments directed opposite the charges of the printing form, are attracted over their entire surface by the printing form; fixing the liquid toner particles with a source of energy in accordance with a picture to be printed; one of removing and breaking down non-fixed liquid toner particles in a manner which changes ink acceptance behavior; and erasing the printing form as a whole, after an end of a printing process, by removing the fixed liquid toner particles.
One particular advantage of the printing form of the present invention is that it can be erased. The fact that the printing form can be illustrated in a printing press is also advantageous. The printing form is preferably developed as a sleeve without a clamping channel on the form cylinder. An electric potential can be applied to the form cylinder so that toner can be applied to the printing form, as in electro-photographic methods.
A further object of the invention is to provide an erasable printing form that can be used in the inventive method.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.